Actuator with an electric motor

ABSTRACT

The present invention relates to an actuating drive including an electronically commutated electric motor which operates with a position-dependent summed torque and which, using a torque controller and a controlled system, is able to be adjusted to a selectable, torque-dependent setpoint value according to a continuously increasing torque-time characteristic curve, the torque controller being able to have supplied to it a torque-dependent actual value of the electric motor in addition to the set point value to derive a control signal. A positioning control is subordinate the torque control, the positioning control providing that the minima between the maxima of the summed torque are rapidly and reliably passed and the electric motor is stopped in the region of a maximum.

BACKGROUND INFORMATION

[0001] The present invention relates to an actuating drive including an electronically commutated electric motor which operates with a position-dependent summed torque and which, using a torque controller and a controlled system, is able to be adjusted to a selectable, torque-dependent setpoint value according to a continuously increasing torque-time characteristic curve, the torque controller being able to have supplied to it a torque-dependent actual value of the electric motor in addition to the set point value to derive a control signal.

[0002] In electric motors with position-dependent motor torque, there is given a lowering of the summed torque exactly in the overlap region of the phases which, from a certain torque level on, can no longer be compensated for by increasing the phase current if the electric motor is designed as a switched reluctance motor. In other motor topologies, the motor torque-position characteristic shows alternating minima and maxima, the maxima occurring at the positions at which only one phase of the electric motor is energized, respectively.

[0003] The minima lie in the overlap regions between two successively energized phases. In this context, both phases are energized, but generate motor torques whose sum is smaller than the maximum motor torque generated by one phase.

[0004] When the position of the motor rotor is slowly and continuously changed, then the stopping position can coincide with a minimum of the motor torque-position characteristic in the adjusted end position. In the case of high load torque and large static friction, the electric motor can no longer reliably start from this position of a minimum.

[0005] Moreover, in the case of a large setpoint torque and a slow change of the torque during the actuating operation, there is a risk for the electric motor to stop while passing through a minimum of the summed torque due to the load torque and the friction.

[0006] The object of the present invention is to guarantee in an actuating drive with electric motor of the type mentioned at the outset that, at all times, an end position is reached which ensures reliable starting of the actuating drive in response to a new control input even in the case of high load torque and large static friction.

[0007] This objective is achieved according to the present invention in that the torque controller emits the control signal as a setpoint position and feeds it to a downstream position controller, that, until a predetermined threshold torque which is smaller than the maximum torque of the electric motor is reached, the setpoint position can be fed unchanged to the position controller and is used together with the actual position of the electric motor to derive the control signal for the controlled system, and that, subsequent to exceeding the threshold torque, the setpoint position can be fed to the position controller as a changed setpoint position until the predetermined setpoint position is reached, the changed setpoint position repeatedly increasing in an abrupt manner according to the increase in the following peak values of the increasing summed torque.

[0008] Using the additional positioning control, it is achieved that the electric motor is always brought to an end position in the region of a maximum of the torque-position characteristic and thus, moreover, is started with nearly maximum motor torque during subsequent restarting. Thus, high load torque and large static friction are reliably overcome and positive starting of the electric motor is guaranteed.

[0009] Moreover, due to the abrupt changes of the changed setpoint position, it is achieved that, from the predetermined threshold torque on, the minima of the summed torque are rapidly and reliably passed as the setpoint values increases, and the next position of the following maximum is adjusted until, finally, the end position which corresponds to the setpoint value is reached.

[0010] A typical application of the actuating drive is given in an electromotively operated brake of a motor vehicle. In this context, the electric motor is started from an operating point which yields a high torque and the braking force is slowly increased further. In this context, the electric motor, which controls the brake caliper, has to close the brake caliper further against the already high return torque of the brake. Using the control according to the present invention, it is possible to increase the end force by about 8% and, at the same, to reduce the current input from the electrical system of the motor vehicle by approximately one third.

[0011] According to one embodiment, the threshold torque is selected to be about 70% of the maximum torque of the electric motor.

[0012] In this context, the further increase of the changed setpoint position subsequent to exceeding the threshold torque is designed in such a manner that after each abrupt increase of the changed setpoint position its value remains constant until the unchanged setpoint position which is associated with the following maximum of the summed torque is reached, and then changes abruptly again.

[0013] Consequently, the changed setpoint position is a function of the position signal of the torque controller, of the predetermined threshold torque, and of the torque-dependent setpoint value.

[0014] The control loop is advantageously designed in such a manner that a computing circuit, which is able to have supplied to it the control signal of the torque controller as a setpoint position and the setpoint value, is inserted between the torque controller and the position controller, and that the computing circuit converts the setpoint position to the changed setpoint position as a function of the setpoint position and the setpoint value, taking into account the threshold torque, and feeds it to the position controller.

[0015] The present invention is explained in greater detail in the light of an exemplary embodiment with reference to the drawings.

[0016]FIG. 1 shows the summed torque of a three-phase, four-pole electric motor as a function of the rotor angle at constant current.

[0017]FIG. 2 depicts the torque-time characteristic curve of the electric motor.

[0018]FIG. 3 illustrates the associated setpoint position as the control signal of the torque controller as a function of time.

[0019]FIG. 4 shows the setpoint position that is changed by a computing circuit as a function of time and torque and fed to the position controller.

[0020]FIG. 5 depicts the control loop of the electric motor, including the torque controller, computing circuit, position controller and the controlled system.

[0021]FIG. 1 shows a position-dependent torque-position characteristic of a switched reluctance motor having 4 rotor poles and 3 phases. In this motor topology, summed torque M_(ss) in the overlap regions between adjacent phases Ph₁ and Ph₂, Ph₃ and Ph₄, etc., shows distinct lowerings, that is, minima which, from a certain torque on, can no longer be compensated for by increasing the phase current. In other motor topologies, for example, 6 rotor poles and 4 phases, the summed torque shows maxima and minima in the overlap region between two phases where only one phase is energized. In any case, the electric motor is one which operates with a position-dependent summed torque.

[0022] In the exemplary embodiment selected, the maxima of summed torque M_(ss) lie in positions P₁ through P₄ in which only one phase Ph₁, Ph₂ or Ph₃ is energized. Between these maxima, there are minima as shown by the torque-position characteristic according to FIG. 1. Plotted threshold torque M_(SW) is selected to be about 70% of maximum peak torque M 100%.

[0023] As FIG. 2 shows, setpoint value M_(setpoint) has associated therewith a continuously increasing setpoint value-time curve which represents the timing during an actuating movement of the actuating drive, that is, of the starting of the electric motor. In this context, predetermined threshold torque M_(SW) is reached after a time t₁.

[0024]FIG. 3 indicates the profile followed by setpoint position ⊖_(setpoint), which is emitted by the torque controller as a control signal for the position controller, as a function of a corresponding time lapse. Finally, FIG. 4 shows how setpoint position ⊖_(setpoint) turns into changed setpoint position {tilde over (⊖)}_(setpoint) from instant t₁ on, that is, from the exceeding of predetermined threshold torque M_(SW) on. When threshold torque M_(SW) is reached, then changed setpoint position {tilde over (⊖)}_(setpoint) changes abruptly to a value which is associated with maximum P₁. In this manner, it is achieved that the following minimum of the summed torque is rapidly and reliably passed. The increased value of changed setpoint position {tilde over (⊖)}_(setpoint) is maintained until position P₂ is reached. Then, the abrupt changing of changed setpoint position {tilde over (⊖)}_(setpoint) is repeated until finally a position P₁, P₂, P₃ or P₄ is reached which corresponds to predetermined setpoint value M_(setpoint) and the electric motor is stopped. In this context, it can be seen how, as the summed torque further increases, the maxima increase as well and determine the abrupt changes in the curve according to FIG. 4.

[0025] The entire control loop for the electric motor is schematically depicted in FIG. 5. Torque-dependent setpoint value M_(setpoint) together with torque-dependent actual value M_(actual), is fed a torque controller RM which emits a setpoint position ⊖_(setpoint) as a control signal to a subsequent computing circuit RS, the control signal following the curve shown in FIG. 3. Setpoint value M_(setpoint) is also fed to computing circuit RS which, taking into account threshold M_(SW), converts the curve to a changed curve for a changed setpoint position {tilde over (⊖)}_(setpoint) according to FIG. 4. A comparison of the curves according to FIGS. 3 and 4 shows that the curve of changed setpoint position {tilde over (⊖)}_(setpoint) remains unchanged compared to the curve of setpoint position ⊖_(setpoint) according to FIG. 3 until threshold torque M_(SW) is reached at instant t₁. After that, however, changed setpoint position e setpoint is changed abruptly, the abrupt change being selected by computing circuit RS in such a manner that it corresponds to the position of the following, increasing maximum of summed torque M_(ss). In this manner, it is guaranteed that the minimum occurring between maxima P₁ and P₂ is rapidly and reliably passed. As the curve according to FIG. 4 shows, this abrupt change of changed setpoint position {tilde over (⊖)}_(setpoint) is repeated each time a minimum is passed until the electric motor is finally stopped in one of positions P₁ through P₄ when setpoint value M_(setpoint) is reached. 

What is claimed is:
 1. An actuating drive including an electronically commutated electric motor which operates with a position-dependent summed torque and which, using a torque controller and a controlled system, is able to be adjusted to a selectable, torque-dependent setpoint value according to a continuously increasing torque-time characteristic curve, the torque controller being able to have supplied to it a torque-dependent actual value of the electric motor in addition to the set point value to derive a control signal, wherein the torque controller (RM) emits the control signal as a setpoint position (⊖_(setpoint)), and feeds it to a downstream position controller (RP); until a predetermined threshold torque (M_(SW)) which is smaller than the maximum torque (M_(ss)) of the electric motor is reached, the setpoint position (⊖_(setpoint)) can be fed unchanged to the position controller (RP) and is used together with the actual position (⊖_(actual)) of the electric motor to derive the control signal (u) for the controlled system (S); and subsequent to exceeding the threshold torque (M_(SW)), the setpoint position (⊖_(setpoint)) can be fed to the position controller (RP) as a changed setpoint position ({tilde over (⊖)}_(setpoint)) until the predetermined setpoint position is reached, the changed setpoint position ({tilde over (⊖)}_(setpoint)) repeatedly increasing in an abrupt manner according to the increase in the following peak values (P1 through P4) of the increasing summed torque.
 2. The actuating drive as recited in claim 1, wherein the threshold torque (M_(SW)) is equivalent to about 70% of the maximum torque of the electric motor.
 3. The actuating drive as recited in claim 1 or 2, wherein after each abrupt increase of the changed setpoint position ({tilde over (⊖)}_(setpoint))/its value remains constant until the unchanged setpoint position (⊖_(setpoint)) which is associated with the following maximum of the summed torque (M_(ss)) is reached, and then changes abruptly again.
 4. The actuating drive as recited in one of the claims 1 through 3, wherein the changed setpoint position ({tilde over (⊖)}_(setpoint)) is a function of the position signal (⊖_(setpoint)) Of the torque controller (RM), of the predetermined torque-dependent setpoint value (M_(setpoint)), and of the predetermined threshold torque (M_(SW)).
 5. The actuating drive as recited in one of the claims 1 through 4, wherein a computing circuit (RS), which is able to have supplied to it the control signal of the torque controller (RM) as a setpoint position (⊖_(setpoint)) and the setpoint value (M_(setpoint)), is inserted between the torque controller (RM) and the position controller (RP); and the computing circuit (RS) converts the setpoint position (⊖_(setpoint)) to the changed setpoint position ({tilde over (⊖)}_(setpoint)) as a function of the setpoint position (⊖_(setpoint)) and the setpoint value (M_(setpoint)), taking into account the threshold torque (M_(SW)), and feeds it to the position controller (RP). 